Natural killer cells (NK) and cytotoxic T cells are lymphocytes with similar effector functions but different times of action during infection. Whereas human NK cells were once considered homogeneous and uninfluenceable by HLA polymorphism, they are now seen to have clonal diversity arising from differential expression of KIR and CD94:NKG2 receptor genes, at least one of which must engage an autologous MHC class I allotype. T-cell subpopulations with activated from memory phenotype also express these "NK-cell receptors." Of the two receptor types, CD94:NKG2 is conserved and shared with mice, whereas KIR is polymorphic, rapidly evolving, not part of the mouse's arsenal and possibly primate specific. The overall goals of this investigation are to understand the nature and specificity of the interactions between HLA class I molecules and NK cell receptors, and the extent to which they determine NK-cell repertoires while maintaining NK-cell self-tolerance. Aim 1 will compare the NK-cell repertoires in sibling pairs defined for HLA and KIR genotype, testing the hypothesis that repertoires are genetically hardwired by these two gene complexes and thus unresponsive to environmental change. Aims 2 and 3 will together test the hypothesis that the current view of KIR specificity for polymorphic HLA-A, B, C determinants is an oversimplification and a potential impediment to future progress. Both aims will employ mutagenesis and a new assay with positive readout to correlate the HLA class I specificity of KIR with structural change, both natural and manipulated, in KIR structure. Aim 2 will study the lineage of KIR3D that provides receptors for HLA-B, with particular focus on KIR3DL polymorphism, and the D0 domain as an indirect modifier that affects the strength of ligand interaction. Aim 3 will similarly study the lineage of KIR2D that provides receptors for HLA-C, also examining the role of the D0 domain. Informing the experimental design of Aims 2 and 3 is appreciation of the similarities and differences between human and chimpanzee MHC class I-specific KIR. The three specific aims define an integrated approach to study, in molecular detail, the interplay between polymorphic HLA class I and polymorphic KIR. Such interplay is likely important, not only for the health of each individual, but also for the health and survival of populations. This research will provide fundamental knowledge essential for future assessment of the role and importance of NK-cell receptor variation in human disease susceptibility and clinical transplantation.